The present invention relates to a golf club head, and specifically relates to a golf club head having an improvement in the thickness of a back surface of a face portion.
Most recent wood clubs have heads, at least the face portions, made of a metal material. A face portion needs to have a sufficiently large thickness to have strength sufficient to withstand impact with a ball. Although the size of heads has been increasing, the size must be less than a volume of 460 cm3 with a tolerance of +10 cm3 by rule. Thus, most drivers now have 460-cm3 large heads extremely close to the upper limit.
Such increase in head size accordingly increases the weight of a face portion. To reduce the weight of a face portion and to retain the strength of the face portion also, the thickness of the face portion is reduced while a rib is provided to a back surface of the face portion. For example, Japanese Patent Application Publication No. 2006-141806 describes a face portion in which six ribs extend from a central portion of the face to circumferential portions of the face.
Meanwhile, if ribs are provided in an X-shaped arrangement on a central portion of a back surface of a face portion, the face portion provides a considerably lower restitution performance when a ball is hit at a sole side or crown side of the face than when at the center of the face. To solve this problem, Japanese Patent Application Publication No. 2008-36050 describes a face portion whose back surface is divided into four segments by such ribs in such a manner that the thicknesses of sole-side and crown-side segments are smaller than those of toe-side and heel-side segments.
Conventionally, a number of high-restitution heads with a remarkably increased coefficient of restitution (hereinafter, COR), have been developed. However, high-restitution heads having a COR of 0.830 or higher cannot be used in competitions after 2008. For this reason, a golf club head having a COR that is not increased at the center of a face portion is currently under development. It has been found, however, that if the COR at the center of a face portion is not increased, the restitution performance at a surface of the face other than the center is reduced; in particular, the performance at a sole side of the face is reduced more than that of a crown side thereof.